Projection cathode-ray tube



Dec. 28, 1948. L. w. PARKER' PROJEGTION CATHOD-RAY TUBE Filed Deo. 19,1946V lll IN VEN TOR.

Patented Dec. 28, 1948 UNITED 4sfr-Ares PATENT orifice PROJECTION`CATHODE-RAY TUBE,

l Louis W. Parker, Linie Neck, N. Y., assigner tc FederalTelecommunication` Laboratories, Inc.,

, New York, N. Y., a corporation of Delaware Application December 19,1946, scri-a1 Nc. 717,224.

I 1 This invention relates more particularly to such tubes which are tobe used` as high intensity fluorescent light sources.

A principal object of the invention is to provide a novel constructionof cathode-ray tube for image projection purposes.

` Another object is to provide a cathode-ray tube of high fluorescentlight intensity by employing a special multiple electron gunconstruction for developing a high current `density in the finalcathode-ray beam without lcorrespondingly increasing the second anodevoltage,` or without correspondingly increasing the electron velocity atthe screen.

A further object is to provide a fluorescent image projection tubewherein a plurality of electron guns are `mounted and correlated With acommon electron baieeleotrode to produce 'what may be termed a singlecomposite virtual cathode of elemental beam cross-section.

A feature of the invention relates to a :cathoderay tube having aplurality of similar electron guns which develop independent beams thatconverge at a common plane Where an apertured electron baille electrodeis provided to'produce a composite virtual cathode of elementalcross-section. The electron image at this baille is then acted upon by amagnetic lens system to produce a focussed image thereof on thefluorescent screen of the cathode-ray tube. y

. Another feature relates to the novel organization and location ofparts which cooperate to provide an improvedcathode-ray tube for thedirect projection ofuorescentimages, such for example as in televisionprojection tubes and the like.

Other features and advantages will be apparent after a consideration ofthe following detailed descriptions arid the appended claims.`

Referring to the drawing which shows in structural schematic and circuitform one preferred embodiment, there is illustrated an evacuatedenvelope I of glass or other material such as customarily employed incathode-raytube construc-v tion. This envelope comprises `a plurality ofdiverging neck portions II, I 2,A which join a com-- mon substantiallycylindrical or tubular body portion I3. Portion I3 joins anf enlarged orbulbous portion I4 which is closed by the usual at` tened window portionI5. Window 'I5 is coated interiorly with a layer I6 of any well-knowniiuorescent material which emits fluorescent light when'boinbarded bycathode rays.

Preferably, the neckportions Il and`I2 are each` arranged at an acuteangle with `respect to 4 creams. (o1. 25oism to cathode-ray tubes andthe Acentral longitudinal axis of the envelope, sol

that the central longitudinal axis of these necks intersectr at theplane I'l-Il. suitably supported Within each neck is anelectron-emitting cath-A ode I8, i9, which is of any Well-known typeused in cathode-ray tubes and comprising for example a tubular metalsleeve Within which is insulatingly supported a heater filament or coil.The end of each sleeve facing the screen I6 is coated withelectron-emissive material to develop a `beam of electrons when theassociated sleeve is raised to a suitable temperature. `Adjacent eachcathode is an apertured control electrode 20, 2l, which. electrodes arearranged to be excited in parallel with variable signals, such forexample as those received .from the video amplifier of any wellknowntelevision receiver. Y

The internal surface oi the envelope between the point 22 and point 23,is coated With a conductive paint such for example as Aquadag orcolloidal graphite 24, which is contacted by a lead-in wire 2.5,sealedvthrough the Wall of the envelope and provided externally with ametal connector cap 26. Cap 20 is connected by conductor 21 to apredetermined point, for example `+1000 volts D. C. in a suitable sourceof D. C. supply. The cathodes I8 and I9 can be suitably biasedpositively with respect to the grounded end of the DQC. source by apotentiometer slider arm 28, and the voltage divider resistor 29. Itwill be observed that a suitable source of D. C. potential,

e. g. 8000 volts, is connected across resistors 29,' 30, 3l, 32 inseries, so that the point 33 can be given any desirable bias, forexample from 0 to 50 volts. The points 34, 35 and 36 are thenrespectively at approximately +1000 volts, +1500 volts and +8000 volts.In the Well-known manner, the coating 24 acts as the rst or acceleratinganode which is common to the two cathoderay beams emitted from thecathode sleeves I8 and I9.

Suitably mounted Within the portion I3 of the envelope and at the planeIl-I'I, is a metal baffle electrode 31, which has a central opening 38,

located on the central longitudinal axis of the tube. This baille 3l isconnected to the point 3'5 which is approximately 500 volts higher inpositive potential than the coating 24. The bulb por-` is any well-knownform of magnetic beam focussing unit 42. Likewise, surrounding the neckportion l2 is a similar beam focusing unit 43. These units 42 and 43 areadjustably mounted so as to change the orientation of the focussingmagnetic field within the respective neck of the tube and therebycorrespondingly control the trajectory of the focussed beam from therespective cathode in such a way that the two beams -converge and arefocussed at the baille opening 38. One particular manner of providingthis adjustability is illustrated in the drawing. For example, thedevice 42 may consist of the usual electromagnetic focussing coil whichis pivotally supported for example at point 44 on a suitable rigidsupport 45, and the opposite side of the coil is adjustably supportedfrom a corresponding xed support 46, through the intermediary of anadjusting screw 41, and a compression spring l48. Thus by turning screwk4I the orientation of the focusing coil 42 with respect to the centrallongitudinal axis of the neck Il, can be readily arranged. Likewise, thefocussing unit 43 is pivotally supported at a point 49 and is providedon the opposite side with an adjusting screw 50 and a correspondingcompression spring 5|. In order to facilitate the focussing of the twobeams 52, 53 at the opening 38, the side of baille 3'I facing thecathodes I8 and I9, is coated with fluorescent material. The screws 47and 50 are adjusted so that the portion of each of the beams whichstrikes the baffle 31 is at a minimum. In the well-known manner, thecurrent through the windings of the focussing coils 42 and 43 can beadjusted to obtain an accurate focus of the beams 52 and 53 at theopening 38. The invention is not limited to an adjustment wherein thecross-section of the converging beams at the plane I1 is less than thesize of the opening 38. If the two beams are improperly focussed, thiswill result merely in a lowering of the brilliancy of the picture on thescreen I6, but will not affect its sharpness. This result is obtainableby reason of the fact that the intersecting beams form in effect avirtual cathode at the opening 38, and the size or crosssection of thisVirtual cathode is xed by the size of the opening 38 regardless of thedegree of focussing by units 42 and 43. Located between the member 31and the screen I6 is any wellknown form of magnetic field producingdevice `54, which acts as an electron lens to form on the screen I6 animage of the virtual cathode at the opening 38. Also located between themagnetic lens 54 and the screen I-B is any wellknown form ofbeam-deecting arrangement 5'5 for deilecting the beam in coordinatedirections so as to scan the screen I6 in the conventional manner.Instead of using electromagnetic focussing coil units 42 and 43, each ofthese units may consist of permanent magnets.

While the drawing shows a tube having two separate electron guns forproducing the intersecting and converging cathode-ray beams, it will beunderstood that a greater number of such guns may be employed. Forexample, if six guns are employed, the tubular body portion lo willterminate in six separate neck portions similar to portions Il and I2,all these portions being symmetrically arranged around the longitudinalcentral axis of the tube so that each gun produces an individualcathode-ray beam which is focussed at the opening 38 to produce thedesired composite virtual cathode thereat.

The above described arrangement has the ad- 4 vantage that it ispossible to obtain an almost unlimited quantity of electrons from thevirtual cathode at the opening 38. According to known theory, themaximum emission from the usual oxide cathode is two milliamperes persquare millimeter. Since in the usual cathode-ray tube, the maximum sizeof the cathode area is limited to 1/4 square millimeters, only about 1/2milliampere is available for cathode current, and the actual beamcurrent may be even less. On the other hand, the brilliancy of thescanning spot on the screen I6 is determined by the watts of powerdissipated on it by the beam. To obtain .as much as 10 watts dissipationon the screen of the usual projection type cathode-ray tube. it isnecessary to use as much as kilovolts and about 330 microamperes beamcurrent. With the arrangement above described, the voltage applied tothe coating 39 may be reduced to 25% of its usual value, for example8000 volts D. C., and the beam current can be increased, depending uponythe number of separate guns that'l are employed for producing the singlevirtual cathode above descirbed. Probably the maximum beam current atwhich well-known saturation effects can be neglected, is about 2milliamperes with 8000. volts electron velocity at the screen. Apractical value would be 8000 volts electron velocity and 1.2milliamperes beam current. This value is not difficult to obtain andproduces the same brilliancy as other projection type tubes whichrequire 30 kilovolts for the second anode potential.

While one particular embodiment has been described herein, it will beunderstood that various changes and modifications may be made Withoutdeparting from the spirit and scope of the invention.

What is claimed is:

1. A cathode-ray tubecomprising an envelope having a fluorescent screen,a plurality of electron guns for developing respective electron beams,an electron baille electrode having a central opening symmetricallydisposed with respect toA said beams, and having a focus-indicatorsurface facing said guns which is coat-ed with fluorescent material tofluoresce when said beams impinge thereon, means to focus said beams atsaid opening to form a virtual composite cathode thereat, and anelectron lens for forming on said screen an image of said compositeVirtual cathode.

2. A cathode-ray tube comprising an envelope having a fluorescentscreen, a plurality of electron guns for developing respective electronbeams, a common electron baille electrode for said beams, said bafflehaving a focus-indicator surface facing said guns which is coated withfluorescent material to fluoresce when said beams impinge thereon,separate forcussingr means for said beams, means to adjust saidfocussing means to focus said beams at the electron opening in saidbaille, and a magnetic lens for imaging on saidgscreen the compositeelectron beam as it passes said baille opening.

3. A cathode-ray tube comprising an envelope having a fluorescentscreen, said envelope having a plurality of diverging neck portions, anelectron gun located in each neck portion to produce respective electronbeams whose trajectories crossover at a point between the guns and thescreen, an apertured electron baille electrode having its aperturelocated at said cross-over point and energized to produce a compositevirtual cathode at said aperture, said baille having a focus-ndicatorsurface facing said guns which is coated with iluorescent material tofluoresce when said beams impinge thereon, and an electron lens forimaging upon said screen the said composite virtual cathode. r

4. A cathode-ray tube comprising an envelope having a fluorescentscreen, said envelope having a plurality of diverging neck portions, anelectron gun located in each neck portion to produce respective electronbeams whose trajectories cross-over at a pointl between `the guns andthe screen, rst and second accelerating anodes for said guns', anapertured electron baille electrode located between said anodes andhaving its aperture located at said cross-over point, said bafe having afocus-indicator surface facing said guns which is coated withfluorescent material to uoresce when said beams impinge thereon, and anelectron lens for imaging upon said screen the composite electron beamas it passes the baille aperture.

LOUIS W. PARKER.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS

